1. Field of the Invention
The present invention pertains to fiber reinforced, brittle matrix inorganic composites, particularly of a cementitious nature, which are lightweight, and which exhibit strain hardening behavior.
2. Background Art
Lightweight concrete has many cases, and is typically produced by incorporating lightweight aggregates such as pyroprocessed shales, clays, slates, expanded slags, expanded fly ash, and aggregates mined from porous volcanic sources into a hydraulically setting matrix. The air-dried unit weight is typically in the range of 1400 to 1900 kg/m3. Yet further decreases in density can be achieved by foaming the mixes, wherein the encased air takes the form of small and spherically-shaped bubbles dispersed within the concrete matrix. P. E. Regan et al., “Lightweight Aggregate Foamed Concrete,” STRUCTURAL ENGINEERING, Vol. 68, No. 9, 1990, pp. 167-173. Expanded polymer beads have also been used in such products.
Lightweight concrete affords the advantage of significant reduction in weight. In bridge and building design, for example, the reduced gravitational load and seismic internal mass allows reduced member sizes and foundation forces. However, use of lightweight concrete is limited due to its lack of ductility. While conventional concrete is hardly known for ductility, the cracking and brittle nature of concrete is even more pronounced in lightweight concrete, because the lightweight aggregate is usually weaker than the cement matrix, and provides little resistance to crack propagation. In such lightweight products, the fracture energy is typically only a fraction of that of normal concrete.
Significant improvement in fracture toughness can be achieved through incorporation of fiber reinforcement. However, conventional fiber reinforced concrete still exhibits quasi-brittle post-peak tension-softening behavior under tensile load, where the bearable load decreases with increases in crack opening. Tensile strain capacity thus remains low, about the same as normal concrete, i.e. about 0.01%. Considerable efforts have been expended into converting the quasi-brittle behavior of fiber reinforced concrete to ductile strain-hardening behavior resembling ductile metals. In most cases, the approach has been to increase the volume fraction of fibers as much as possible. As the volume fraction of fibers reaches the range of 4-10%, depending on the particular fiber, moderate strain hardening behavior may be achieved. For example, WO 99/58468 discloses a high performance concrete of normal density, wherein moderate strain hardening is achieved by incorporation of 4% by volume of polyvinyl alcohol fibers in a compact matrix which includes very hard, small diameter fillers. Nevertheless, the strain capacity is still less than 0.5%.
High fiber volumes create processing problems, however. Homogenous fiber dispersion become difficult due to the increasing viscosity of the mix as more and more fibers are dispersed. The high fiber surface area and the mechanical interaction between the fibers raises the viscosity such that processing also becomes difficult. Various processing techniques have been disclosed in attempts to solve such problems, such as the extrusion process of U.S. Pat. No. 5,891,374.
Lightweight concrete, as mentioned earlier, exhibits even higher brittleness and lower compressive strength than concrete of normal density. Adding large amounts of reinforcing fibers to concrete mix with lightweight aggregates is problematic, since the lightweight aggregate interferes with uniform dispersion of reinforcing fibers. Several attempts have been made to create lightweight concrete with improved tensile strain and strain hardening capacity. However, such attempts have not proven successful, due to inappropriate fiber type and weak interface bond strength. Moreover, the importance of size control of lightweight fillers has not been recognized by the art.
In U.S. Pat. No. 4,407,676, polyolefin film with fibrillated, hooked microfibrils is used to reinforce cement mortar containing gas bubbles or cellular plastic chunks. As cracks develop, bridging forces are provided predominantly by the mechanical interlock between the hooked microfibrils and the mortar matrix. The fibrillating process, however, provides various fibril dimensions. The ultimate composite tensile strength is limited as the small diameter fibers are ruptured under strain due to low fiber strength, and large diameter fibers are easily pulled from the matrix due to low bond strength and high Poisson ratio of polyolefin fibers. The resulting lightweight concrete does not exhibit tensile strain hardening, and compressive strength remains low, due to the use of gas bubbles or cellular plastic chunks.
U.S. Pat. No. 5,002,620 discloses multilayer concrete structures, where lightweight gas-bubble-containing, fiber reinforced concrete is used in conjunction with a dense, fiber reinforced concrete. Polypropylene fibers and carbon fibers are disclosed as being useful. However, carbon fibers are ill suited for use in cementitious composites when randomly distributed due to their brittleness and vulnerability to bending stress. Moreover, the structures disclosed are not only highly anisotropic, but also cannot, in tandem, achieve the strain hardening behavior desired of lightweight concrete.
U.S. Pat. No. 5,030,282 discloses the use of unidirectionally oriented continuous carbon fibers at high volume percent concentration to achieve light weight. However, such composites require unique processing, and are also highly anisotropic.
U.S. Pat. No. 6,203,609 discloses lightweight concrete containing polypropylene fibers and gas bubbles produced by reaction between aluminum metal powder and alkaline cementitious ingredients. However, strain hardening behavior is not achievable.
It would be desirable to provide lightweight concrete materials with higher physical properties than those presently available, in particular, high fracture toughness and strain hardening capacity, while maintaining light weight, ease of processing and application, and physical properties such as compressive strength.